In prior art transflective liquid crystal displays (LCDs) have been reported. These displays can be operated both in transmissive mode, where light generated from a backlight is transmitted through the switchable LC cell towards the viewer, and reflective mode, where ambient light is reflected by a reflector behind the switchable LC cell and redirected through the LC cell towards the viewer. Because the backlight is needed only in dark ambience, these displays have lower power consumption than standard backlit displays. They are especially suitable for mobile applications, such as mobile phones, notebooks or palmtop devices like PDAs (personal digital assistant).
Kubo et al., IDW 1999, page 183-187, Roosendaal et al., SID Digest 2003, page 78-81 and WO 03/019276 A2 disclose active-matrix (AM) type transflective displays (or “Advanced TFT”), wherein each pixel is divided into a reflective and a transmissive subpixel. The transmissive subpixel has transparent electrodes that transmit light from a backlighting system into the switchable LC medium. The reflective subpixel has a transmissive front electrode and a reflective back electrode that reflects ambient light back towards the switchable LC medium. This type of patterned electrode structure can be achieved for example by “hole in mirror” technology. In the reflective subpixels light passes the LC layer twice so that it has a longer optical path and experiences higher retardation than in the transmissive subpixel, leading to different optical characteristics in the subpixels. Therefore it was proposed to reduce the LC cell gap in the reflective subpixel to approximately half the cell gap of the transmissive subpixel, so that both subpixels provide about the same total phase retardation. This is reported to improve transmission and backlight efficiency.
Roosendaal et al. and WO 03/109276 A2 further suggest to use a patterned quarter wave film (QWF) retarder to improve the viewing angle, brightness and efficiency. The QWF is patterned such it essentially covers only the reflective subpixels.
WO 03/019276 A2 and B. van der Zande et al., SID Digest 2003, page 194-197 disclose several methods to provide such a patterned QWF. According to a first method, a layer of an oriented reactive LC material is photopolymerised through a photomask and the non-polymerised material (in the covered parts of the layer) is afterwards removed, leaving only regions of oriented LC polymer material having the desired birefringence (and thus optical retardation). According to a second method, a layer of oriented reactive LC material is photopolymerised in two steps at different temperatures, where in the first step a photomask is used and the temperature is within the nematic phase of the LC material, and in the second step no photomask is used and the temperature is above the clearing point of the LC material, leaving regions having the desired birefringence and regions having no birefringence. According to a third method, a layer of oriented reactive LC material is provided on a patterned alignment layer inducing different orientation direction of the LC material which is then fixed by photopolymerisation. For example, the orientation direction in the regions serving as QWF is planar (i.e. parallel to the film plane) and at an angle of 45° to the transmission axis of the polarizer, whereas the orientation direction in the regions not serving as QWF is homeotropic (i.e. perpendicular to the film plane) or planar but parallel to the transmission axis of the polarizer.
However, the above methods have several disadvantages. Thus, the removal of non-polymerised material according to the first method requires an additional process step. Although it is not exactly disclosed in WO 03/019276 how the material can be removed, it is likely that additional reagents or mechanical will be necessary. The polmyerisation at different temperatures according to the second method requires temperature variation and control during the film manufacturing process. The third method requires to produce e.g. both regions with accurate uniform planar and homeotropic alignment in one film which is a difficult process. Thus, it is necessary to accurately create a patterned alignment layer, for example by creating uniform, large area surface gratings, creating patterned high/low surface energy substrates, or using photolithographic techniques, all of which imply complicated and costly manufacturing methods and materials. Another disadvantage is that defect alignment (e.g. splayed) can occur at the interface between the planar and homeotropic regions.
One aim of the present invention is to provide a patterned retardation film for use in transflective LCDs, especially in active matrix colour LCDs, which does not have the drawbacks of prior art films mentioned above, allows efficient conversion between linear and circular polarised states for light of different wavelengths and can be prepared by an easy method enabling accurate control of the optical properties. Another aim is to provide advantageous methods and materials for the preparation of such a retardation film. Another aim is to provide an improved transflective display comprising such a patterned retardation film. Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
The inventors have found that these aims can be achieved by providing displays, retardation films and methods according to the present invention.